1. Field of the Invention
The present invention relates to a carbon-containing lithium-iron composite phosphorus oxide for a positive electrode active material, which can constitute lithium secondary batteries in which the dope-undope phenomenon of lithium is utilized, and moreover to a process for producing the same.
2. Description of the Related Art
As personal computers, video cameras, cellular phones, and the like, have been downsized, lithium secondary batteries have been put to practical use. In the fields of information-related appliances and communication appliances, the lithium secondary batteries have been spread widely as a power source, which is employed for the appliances, due to the reason that they exhibit high energy densities. While, in the field of automobiles as well, it has been urged to develop electric automobiles because of the environmental problems as well as the resource problems. As a power source for electric automobiles as well, lithium secondary batteries have been investigated.
At present, as positive electrode active materials of lithium secondary batteries, which can constitute 4V-class secondary batteries, lithium-transition metal composite oxides, such as LiCoO2, LiNiO2 and LiMn2O4, have been used preferably. Especially, not only since LiCoO2 can be readily synthesized and can be handled with relative readiness, but also since it is good in terms of the charge-and-discharge characteristic, lithium secondary batteries, which use LiCoO2 as the positive electrode active material, are one of the main streams.
However, cobalt, and so on, are less in view of the resource amount. Accordingly, the lithium secondary batteries, which use LiCoO2, etc., as the positive electrode active material, are less likely to cope with the future mass-producing and upsizing that aim at batteries for electric automobiles. Moreover, such lithium secondary batteries should inevitably be expensive extremely in view of the cost. Hence, trials have been made in which lithium-iron composite oxides, which contain iron as a constituent element, are used instead of cobalt, etc., because iron is abundant as the resource and because it is less expensive.
As one of the trials, Japanese Unexamined Patent Publication (KOKAI) No. 9-134,725, for instance, discloses lithium secondary batteries in which LiFePO4, LiFeVO4, and the like, having an olivine structure, are used as the positive electrode active material.
However, according to the re-tests which were carried out by the inventors of the present invention, it was not possible to obtain a sufficient capacity by the lithium secondary batteries, which used the lithium-iron composite oxides, having an olivine structure as set forth in the above-described publication, as the positive electrode active material. Namely, it is believed that 90 mAh/g or more is required as an active electrode discharge capacity when lithium secondary batteries are charged and discharged at a practical charge-and-discharge density. However, when the aforementioned lithium-iron composite oxides were used as the positive electrode active material, it was difficult to obtain such an active material discharge capacity. Moreover, it was found out that the lithium secondary batteries, which used the aforementioned LiFePO4, etc., as the positive electrode active material, exhibited the lowering active material discharge capacity when they were charged and discharged repeatedly, namely, they suffered greatly from the so-called cycle deterioration.